DRILLING INTO AN ACTIVE HYDROTHERMAL SYSTEM AT SUIYO SEAMOUNT OF THE IZU-BONIN ARC, WESTERN PACIFIC

Shallow drilling (up to 12 m) using a tethered Benthic Multicoring System (BMS), was conducted within the Suiyo Seamount active hydrothermal field (28°34'N, 140°30'E, depth 1360m) as part of the Archaean Park Project during 2001 and 2002. Several fluid samples were collected from both active vents and the drilled holes during dive programs using the ROV Hakuyo 2000 and DSRV Shinkai 2000. Here we report a summary of these investigations focusing on geochemical characteristics of hydrothermal systems developed within an arc volcano.

Hydrothermal fluid circulation system develops within both volcaniclastic sediments and underlying dacitic lava/pyroclastic rock on the caldera floor. The active vent field covers 250 m by 200 m. In the central region, high temperature (~300°C) smoker chimneys coalesce to form somewhat thick cluster of hydrothermal mounds on the seafloor. The chemical composition of the hydrothermal fluid samples can be interpreted as simple mixing between a unique hydrothermal endmember and ambient seawater, irrespective of a wide range in sampling sites and fluid temperatures, indicative of a single fluid reservoir beneath the caldera floor. As some of the drill holes intersected the reservoir, we believe it extends at a few meters below the seafloor, at least in some locations.

Among the drilled cores from the central region, it is notable that mixture of anhydrite sericite, pyrite and quartz forms "cap-rock" which bounds the top of the reservoir. In deeper parts of the reservoir, mixed-layer chlorite/montromollonite, mica and chlorite were observed. Formational temperature of the mica and chlorite are calculated to be 250 to 290°C, based on oxygen isotope ratios. Chemical equilibrium of fluid-mineral interactions within the hydrothermal reservoir is considered the controlling factor of the major element composition of the hydrothermal fluid. Another important feature is significant contributions of magmatic volatiles. High CO₂ concentrations could control fluid pH by calcite saturation. Magmatic volatiles could transport various metals into the hydrothermal system. Hydrothermal systems in the intraoceanic arc setting provide suitable conditions for the formation of hydrothermal ore deposits.